1. Field of the Invention
The present invention relates to a method for producing an aerosol stream. The invention relates, in particular, to the production of an aerosol stream containing glass forming SiO.sub.2 particles suitable for the production of a preform for optical fibers.
2. Technology Review
U.S. Pat. No. 4,597,983 by Kuhne discloses, for example, the production of an aerosol stream by means of a flameless chemical reaction of the gaseous and/or vaporous components silicon tetrachloride (SiCl.sub.4) and water (H.sub.2 O) according to the following formula : EQU SiCl.sub.4 +2 H.sub.2 O.fwdarw.SiO.sub.2 +4 HCl
It is further known to use additional components, e.g. GeCl.sub.4, to obtain doping of the resulting SiO.sub.2 particles. The result is, for example, germanium dioxide according to the following formula: EQU GeCl.sub.4 +2 H.sub.2 O.fwdarw.GeO.sub.2 +4 HCl
These chemical reactions take place in a heatable reaction chamber which has, for example, a tubular configuration or is an open burner. In the latter case, it is advisable to preheat the reacting components and/or the burner so that the required reaction temperature exists at the point where the components exit.
These methods have the common feature that the components are initially supplied separately, e.g. in concentrically arranged pipes, to point of entry into the reaction chamber in which the aerosol stream is created. Particularly for the production of preforms for optical fibers, it is necessary for the aerosol stream to have as uniform as possible a concentration of SiO.sub.2 particles over its cross-sectional area and is additionally a laminar flow.
In general, a uniform aerosol stream requires a uniform, laminar gas flow of all components and their most complete mixing possible. A laminar stream usually requires slow gas flow velocities and careful configuration of the point of entry into the reaction chamber. Mixing results from diffusion perpendicularly to the direction of gas flow.
However, these prior methods have the drawback that the components are mixed through so quickly that, in an undesirable manner, a reaction product is formed at the point of entry and this reaction product is not transported along with the aerosol stream in the form of small particles but instead is deposited as a solid at the point of entry. Consequently there occurs a mechanical change which interferes in an unpredictable manner with the uniformity of flow. It is even possible that the point of entry may become clogged.
These drawbacks can be overcome by a high flow velocity which quickly transports the components away from the point of entry before they are mixed through. However, the flow velocities required for this purpose are so high that the desired laminar flow changes to an undesirable turbulent flow. This disadvantage can be avoided by spatially separating the components at the point of entry, for example with the aid of an inert gas conducted between the components. This extends the diffusion path so that even with slow flow velocities, mixing and reaction between the components do not occur until some distance from the point of entry. However, the amount of apparatus required for such an arrangement is uneconomically high particularly for industrial production, primarily if the resulting aerosol must have a high degree of purity.